1. Technical Field of the Invention
The present invention relates to a method for welding a base material and a deposit metal, and particularly relates to a welding method for reducing the residual stress after welding a deposit metal to a base material, and increasing the fatigue strength of a base material and welded joint portion.
2. Conventional Art
Herebelow, a summary of the conventional art shall be described with the welding of a bond cable for a railroad rail. FIG. 1 schematically illustrates a joint portion of a railroad rail. While the rail 100 which is anchored to a tie 120 by means of a spike 110 is such that a predetermined gap 130 with an adjacent rail is left in order to allow for thermal expansion, they are connected to each other electrically by means of bond cables 140. This is because the rails 100 are also used as channels for passing the drive current and control signals of railway cars.
FIG. 2 is a perspective view showing an enlargement of the area of attachment of the bond cables 140. In the example shown, the end portion 150 of the bond cable is welded to a web on the rail 100 by means of a deposit metal 160. The procedure for welding the deposit metal 160 to the web on the rail 100 is performed on-site after the rail 100 has been lain. Examples of methods for performing the weld include many types such as low-temperature wax welding, arc welding and others, with many procedures having been proposed, but of these, copper thermit welding has the highest joint strength at the welding portion yet does not require high levels of skill, and has consequently come into popular use in recent years.
Thermit welding is a method wherein a mixed powder of aluminum and copper oxide (or iron oxide) is ignited near the surface of the base material, to melt and weld the copper (or iron) and a portion of the base material with the heat of the chemical reaction.
Although thermit welding is a welding method which is convenient and can achieve high strength as described above, it leaves residual stress in the area of the weld, thus decreasing the fatigue strength. For example, as described in Miki et al.,  less than  less than Methods of Increasing Fatigue Strength by Improvements to Weld Toe Portion greater than  greater than , Journal of Japan Welding Society, vol. 17, no. 1, reduction of the stress concentration at the weld toe portion and reduction of tensile residual stress caused by welding are known to be effective for improving the fatigue strength of weld joints. Additionally, this paper also describes that hammer peening, in addition to TIG treatments and grinder treatments, have some effect as methods for relieving this stress.
Hammer peening is a method in which the weld portion is struck by a hammer to plastically deform the base material, and the tensile residual strength of the weld is relieved by the residual strength on the pressed side caused by this deformation, so as to consequently lessen the decrease in the fatigue strength caused by the weld. As described in the paper, in the hammer peening process, the blows are struck at the peripheral portions of the weld where the residual tensile strength is believed to be highest. While this hammer peening process can raise the fatigue strength to some extent, there are reports that fatigue cracks can develop from scratches formed in the base material during the hammer peening process.